The present invention generally relates to mechanical heart valve prostheses. More specifically, the present invention relates to improved bileaflet mechanical heart valve prostheses.
Prosthetic valves are utilized to replace malformed, damaged, diseased or otherwise malfunctioning valves in body passageways, such as heart valves including the tricuspid valve, the bicuspid or mitral valve, the aortic valve and the pulmonary valve. Such prosthetic heart valves are typically implanted into the heart either by open chest surgery which requires a sternotomy or by minimally invasive surgery which requires a thoracotomy between adjacent ribs.
Heart valve prostheses may be divided into two groups, namely tissue valves and mechanical valves. Prosthetic tissue valves are harvested from a suitable animal heart, usually a porcine heart, prepared according to known methods, and may be mounted to a stent to facilitate implantation. Mechanical valves, by contrast, utilize a synthetic valve having a ball, a disc, a pair of leaflets or occluders (bileaflet), or a plurality of leaflets to regulate blood flow therethrough.
A mechanical heart valve prosthesis is optimally designed to perform the same functions as a healthy native valve under the same operating conditions. In particular, a mechanical heart valve is designed to regulate blood flow into and out of the heart chambers. Mechanical heart valves permit blood flow in only one direction and are actuated between an open position and a closed position by the changing hemodynamic conditions of the heart--i.e., by changes in blood flow and pressure caused by the pumping action of the heart.
Ideally, a mechanical heart valve prosthesis imposes no more resistance to blood flow than a healthy native heart valve. However, mechanical valves typically have less efficient flow and may be more thrombogenic than healthy native valves. The inefficient flow may be caused by design limitations associated with the relatively small orifice, the profile or shape of the leaflets and the dynamic movement of the leaflets. The potential consequence is a high pressure drop and/or turbulent flow across the valve. The thrombogenic nature is usually due to the valve geometry which may include stagnation points and the valve material which may have some inherent thrombogenic properties. The dynamic movement of the leaflets at valve closing generates a high impact force which damages the blood elements and further contributes to the thrombogenicity of a valve design. Improvements in blood flow efficiency and thrombogenic resistance are desirable to more closely simulate a healthy native valve.
As stated previously, inefficient flow in mechanical heart valves may be caused by the relatively small orifice--i.e., the relatively small opening through which blood flows when the valve is in the open position. The size of the orifice is limited by the space consumed by the sewing or suture ring (cuff) and the valve housing. The suture ring is necessary to facilitate mounting the valve in the heart and the valve housing is necessary to support the occluders.
The valve housing typically defines a flow orifice(s). In a bileaflet valve design, two opposing flat portions are typically utilized to accommodate the hinge points, with two hinge points associated with each flat portion. The two flat portions consume additional space that might otherwise define the orifice opening and accommodate more blood flow. It is desirable, therefore, to minimize the effect of the flat portions in a bileaflet valve design in order to improve the efficiency of the valve.
Also in a bileaflet design, it is desirable to maximize the size of the central opening--i.e., the opening between the two leaflets when the leaflets are in the open position. Maximizing the central opening reduces drag caused by the leaflets and improves blood flow in the center of the lumen which is typically the high velocity and high fluid stress region. One approach to increase the size of the central opening is by spacing the axes of the leaflets further apart. This typically requires an enlargement of the flat portions on the housing to accommodate the distance between the pivot axes. Increasing the size of the flat portions is undesirable because the flow area is reduced, as discussed previously. Accordingly, it is desirable to increase the size of the central opening in a bileaflet valve, without sacrificing the efficiency in utilizing the total orifice area. Another consideration in a mechanical heart valve design is the dynamic movement of the leaflets at valve closing. A lower leaflet closing velocity (soft closing) is desirable because it generates a lower closing impact force and minimizes damage to blood elements.